Image printing devices require precise measurements of internal moving parts and image receiving mediums in order to produce accurate images. Optical encoders have traditionally been employed to monitor the moving parts of image printing devices assuring correct placement of an image being formed on an image receiving medium. An optical encoder is a device that detects and measures movement (either linear or rotary) through the use of one or more photosensor elements. In order to measure the movement of a selected device, a reference object is formed having a known repetitive pattern of reflective and non-reflective regions that can be detected by the photosensor elements. When there is relative motion between the reference object and the photosensor elements, the repetitive pattern passes through an illuminated area and the light is modulated by the reflective and non-reflective regions. This modulated light is detected by the photosensor elements at a rate proportional to the rate of relative motion between the encoder and the reference object.
The above-mentioned method has traditionally been used to detect and measure the position of print heads in ink-jet image forming devices. An encoder assembly would be secured to a print head while a patterned strip is placed on a stationary object near the path of the print head. When the print head moved relative to the patterned strip, the repetitive pattern would modulate light that could subsequently be detected by photosensor elements at a rate proportional to the rate of linear movement of the print head. The photosensor elements, in turn, would output a signal indicative of the linear movement of the print head which could then be used to control the linear rate or position of the print head.
The traditional use of patterned targets requires strict adherence to encoder specifications in order to assure proper encoder accuracy. Moreover, numerous manufacturing steps and multiple parts are required for proper encoder use within an image forming device increasing the cost and difficulty of manufacturing.